The present invention relates to vehicle navigation systems and, more particularly, to vehicle navigation systems that employ positioning data from the Global Positioning System (GPS) as an input.
Use of the Global Positioning System ("GPS") has been a principal method for vehicle navigation systems to sense an absolute position of the vehicle. However, the accuracy of GPS suffers from the current incomplete coverage of its satellites, from problems of multiple path reflections, and from artificial accuracy degradation caused by the so-called Selective Availability that results from military control of GPS. Experience demonstrates that, although the root-mean-square (RMS) error of GPS is generally 30 meters, transient errors of up to several hundred meters are not unknown.
The error in a position found from GPS can be divided into two components. The first component is the short-term effect caused by the temporary interference of buildings or trees with line-of-sight transmission from satellites. The second component is the long-term effect caused by poor satellite geometry.
Many current vehicle navigation systems use GPS as one part of a hybrid system. The other part uses dead-reckoning sensors to verify the validity of GPS signals. When GPS signals are determined to be unreliable, a switching algorithm switches the system to positions determined by dead reckoning and ignores GPS positions. Conversely, when the signals from GPS are determined to be valid, GPS positions alone are used and dead-reckoning positions are ignored.
Hitherto hybrid navigation systems have largely confined themselves to detecting problems with positions from GPS rather than to correcting them when detected. Most systems simply switch to another source of positioning information if they determine that GPS positions are unreliable. However, there is no guarantee that the reference source is any more reliable than GPS. The reference source may be in error, or both it and GPS may be in error.
U.S. Pat. No. 4,837,700 discloses a system wherein an odometer is used to determine whether the vehicle is stopped. If the vehicle is stopped, its average GPS position is processed to reset the vehicle's location.
U.S. Pat. No. 4,899,285 discloses a system that compares GPS positions and signal characteristics with those from dead-reckoning sensors. If the GPS position is determined to be reliable, it is used to reset the dead-reckoning position.
U.S. Pat. No. 4,899,285 discloses a similar system. Vehicle velocity and/or acceleration generated from GPS position measurements is compared with dead reckoning. If the comparison shows them to give similar results, the GPS position is used. If not, the dead-reckoning position is used.
However, the error in GPS positioning caused by poor satellite geometry usually varies slowly over time. The result is an offset in the GPS position that also varies slowly over time. Such an offset cannot in general be detected by checking the difference in heading, velocity, or acceleration between the GPS and the dead-reckoning sensors.
Moreover, the dead-reckoning position itself is calculated by integrating the relative displacement vectors generated by the dead-reckoning sensors. Hence the accumulation of errors over time cannot be avoided. Therefore dead-reckoning positions are not absolutely accurate, and there is no way to choose which positioning source, GPS or dead reckoning, is in error. In most cases both are.
Thus the only absolute positioning available to a navigation system comes from a database of roads. Most road databases are digitized from highly accurate maps, from aerial photographs, or from ground surveys. Their margin of error can be as close as 50 feet, so that much greater accuracy is available from a road database than from the 100 feet root-mean-square error and several hundred meters transient error of GPS.